Light directing film

ABSTRACT

The present invention discloses a light directing film comprising a first structured major surface, a second major surface opposite to the first structured major surface, wherein the first structured major surface comprises a first prism element and a second prism element meandering in a wary manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/171,645 filed on Feb. 3, 2014, which is a continuation ofU.S. patent application Ser. No. 12/854,815 filed on Aug. 11, 2010, nowU.S. Pat. No. 8,641,259, which claims (a) priority of U.S. provisionalapplication Ser. No. 61/233,040 filed on Aug. 11, 2009, and is: (b) acontinuation-in-part of U.S. patent application Ser. No. 11/825,139filed on Jul. 2, 2007, now U.S. Pat. No. 7,883,647, which claimspriority of U.S. provisional application Ser. No. 60/818,044 filed onJun. 30, 2006; (c) a continuation-in-part of U.S. patent applicationSer. No. 12/455,021 filed on May 26, 2009, now U.S. Pat. No. 8,899,768,which claims priority of U.S. provisional application Ser. No.61/128,813 filed on May 23, 2008; (d) a continuation-in-part of U.S.patent application Ser. No. 12/590,855 filed on Nov. 12, 2009, now U.S.Pat. No. 8,517,573, which is a continuation of U.S. patent applicationSer. No. 11/450,145, filed Jun. 9, 2006, now U.S. Pat. No. 7,618,164,which claims priority of U.S. provisional application Ser. No.60/689,650 filed on Jun. 9, 2005; and (e) a continuation-in-part of U.S.patent application Ser. No. 12/832,021 filed on Jul. 7, 2010, now U.S.Pat. No. 8,595,964, which claims (e1) priority of U.S. provisionalapplication Ser. No. 61/223,388 filed on Jul. 7, 2009 and is (e2) acontinuation-in-part of U.S. patent application Ser. No. 12/590,855filed on Nov. 12, 2009, now U.S. Pat. No. 8,517,573, which is acontinuation of U.S. patent application Ser. No. 11/450,145, filed Jun.9, 2006, now U.S. Pat. No. 7,618,164, which claims priority of U.S.provisional application Ser. No. 60/689,650 filed on Jun. 9, 2005.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 14/729,029 filed on Jun. 2, 2015, which is acontinuation-in-part of U.S. patent application Ser. No. 12/590,855filed on Nov. 12, 2009, now U.S. Pat. No. 8,517,573, which is acontinuation of U.S. patent application Ser. No. 11/450,145, filed Jun.9, 2006, now U.S. Pat. No. 7,618,164, which claims priority of U.S.provisional application Ser. No. 60/689,650 filed on Jun. 9, 2005.

All of these applications are incorporated by referenced herein in theirentirety.

BACKGROUND

1. Field of Invention

The present invention relates to optical substrates having a structuredsurface, particularly to optical substrates for brightness enhancement,and more particularly to brightness enhancement substrates for use inflat panel displays having a planar light source.

2. Description of Related Art

Flat panel display technology is commonly used in television displays,computer displays, and displays in handheld electronics (e.g., cellularphones, personal digital assistants (PDAs), etc.). Liquid crystaldisplay (LCD) is a type of flat panel display, which deploys a liquidcrystal (LC) module having an array of pixels to render an image.

FIG. 1 illustrates an example of an LCD display (which may be modifiedto include the optical substrate in accordance with the presentinvention). A backlight LCD 10 comprises a liquid crystal (LC) displaymodule 12, a planar light source in the form of a backlight module 14,and a number of optical films interposed between the LC module 12 andthe backlight module 14. The LC module 12 comprises liquid crystalssandwiched between two transparent substrates, and control circuitrydefining a two-dimensional array of pixels. The backlight module 14provides planar light distribution, either of the backlit type in whichthe light source extends over a plane, or of the edge-lit type as shownin FIG. 1, in which a linear light source 16 is provided at an edge of alight guide 18. A reflector 20 is provided to direct light from thelinear light source 16 through the edge of the light guide 18 into thelight guide 18. The light guide is structured (e.g., with a taperedplate and light reflective and/or scattering surfaces defined on thebottom surface facing away from the LC module 12) to distribute anddirect light through the top planar surface facing towards LC module 12.The optical films may include upper and lower diffuser films 22 and 24that diffuse light from the planar surface of the light guide 18. Theoptical films further includes upper and lower structured surface,optical substrates 26 and 28 in accordance with the present invention,which redistribute the light passing through such that the distributionof the light exiting the films is directed more along the normal to thesurface of the films. The optical substrates 26 and 28 are oftenreferred in the art as luminance or brightness enhancement films, lightredirecting films, and directional diffusing films. The light enteringthe LC module 12 through such a combination of optical films is uniformspatially over the planar area of the LC module 12 and has relativelystrong normal light intensity. The LCD 10 may be deployed for displays,for example, for televisions, notebook computers, monitors, and portabledevices such as cell phones, PDAs, cameras, and the like.

There is an increasing need for reducing power consumption, thicknessand weight of LCDs, without compromising display quality of the LCDs.Accordingly, there is a need to reduce power consumption, weight andthickness of backlight modules, as well as thicknesses of the variousoptical films. In this regard, many light directing techniques have beendeveloped to reduce power consumption without compromising displaybrightness. Some developments are directed to the design of thebacklight module (i.e., designing structures of the components of thebacklight module 14 in FIG. 1, comprising the light source 16 andreflector 20, and light guide 18, to improve overall light outputperformance. In addition, other developments are directed to diffuserfilms 22 and 24, and luminance/brightness enhancement films 26 and 28.

In the backlight LCD 10, brightness enhancement films 26 and 28 useprismatic structures to direct light along the viewing axes (i.e.,normal to the display), which enhances the brightness of the lightviewed by the user of the display and which allows the system to useless power to create a desired level of on-axis illumination.Heretofore, brightness enhancement films were provided with parallelprismatic grooves, lenticular grooves, or pyramids on the light emittingsurface of the films, which change the angle of the film/air interfacefor light rays exiting the films and cause light incident obliquely atthe other surface of the films to be redistributed in a direction morenormal to the exit surface of the films. The brightness enhancementfilms have a light input surface that is smooth, through which lightenters from the backlight module. Heretofore, many LCDs used twobrightness enhancement film layers (as in the LCD in FIG. 1) that arerotated about an axis perpendicular to the plane of the films, relativeto each other such that the grooves in the respective film layers are at90 degrees relative to each other, thereby collimating light along twoplanes orthogonal to the light output surface.

Heretofore, much effort have been undertaken to develop the structuredsurface of the brightness enhancement films. FIG. 2 illustrates thestructures of various prior art brightness enhancement films. The lightoutput surfaces of the brightness enhancement films (the top surface asshown in the figures) are structured, and the light input surfaces (thebottom surface as shown in the figures) are flat and smooth (e.g.,glossy). When the brightness enhancement films are used in LCDs, withthe glossy bottom surface of a brightness enhancement film above thestructured surface of another brightness enhancement film, it has beenexperienced that the optical interaction between the glossy surface oftop enhancement film and the structured surface and/or glossy surface ofthe lower brightness enhancement film creates undesirable visibleartifacts in the display image in the form of interference fringes(i.e., bright and dark repeated patterns) that are observable in thedisplay image. Undesirable image affecting effects arising frominterference fringes, physical defects, flows, stains andnon-uniformities, etc., can be masked by using an upper diffuser film(e.g., diffuser film 22 above brightness enhancement film 26 in FIG. 1).

Heretofore, to reduce the overall thickness of the optical films inLCDs, much effort had been directed to reducing the number of theoptical films, from four films (e.g., optical films 22, 24, 26 and 28 inFIG. 1) to three films. In this regard, typically the low diffuser film24 and low brightness enhancement film 28 are maintained as separatestructures, but the functions of the top diffuser film 22 and topbrightness enhancement film 26 are combined and merged into a singlehybrid film structure. The three-film type display has been widelyadopted in handheld electronic devices and notebooks, where it isparticularly desirable to push the envelope to reduce overall size ofsuch devices.

Various efforts have been undertaken to develop hybrid brightnessenhancement films. Referring to FIG. 3, U.S. Pat. No. 5,995,288disclosed a coating layer of particles provided on the underside of theoptical substrate, on the opposite side of the substrate with respect tothe structured surface on the top side. Referring to FIG. 4, U.S. Pat.No. 5,598,280 disclosed a method to form small projections the undersideof the optical substrate to improve uniformity in luminance. Others haveexplored modifying the structure of prism surface of the structuredsurface of the optical substrate. For example, referring to FIG. 5B,U.S. Pat. No. 6,798,574 provides fine protrusions on the prism surfaceof the structured surface of the optical substrate, which is supposed todiffuse light in a certain direction with a wider angle.

However, the above-mentioned hybrid brightness enhancement films involverelatively complex structures requiring relatively higher manufacturingcosts. Moreover, the hybrid brightness enhancement films are also lesseffective in directing light within the desirable viewing angle.

Furthermore, in the absence of a separate top diffuser film between thestructured surface of the top hybrid brightness enhancement film and theunderside of the LC module, undesirable interference fringes appearingas bright and dark patterns may be generated. It is known that the topstructures on the brightness enhancement film and the pixel array in theLC module could create interference fringes or moire patterns as well.

There remains a need for a cost effective optical substrate thatprovides a surface structure that enhances brightness and reducesinterference fringes, whether used with another brightness enhancementfilm or a LC module.

SUMMARY OF THE INVENTION

The present invention is directed to an optical substrate that possessesa structured surface that enhances luminance or brightness and reducesinterference fringes in the display images. In one aspect of the presentinvention, the optical substrate is in the form of a film, sheet, plate,and the like, which may be flexible or rigid, having a structured lightoutput surface that comprises rows of laterally arranged snaking, wavyor meandering longitudinal prism structures. In one embodiment, theprism structures at the light output surface may be viewed as comprisingrows of laterally meandering longitudinal prisms and/or sections ofcontinuous curved segments (i.e., sections with a curve in a particulardirection, or generally C-shaped curve sections) coupled end-to-end toform the overall meandering longitudinal prism structures. In oneembodiment, the laterally meandering rows of longitudinal prismstructures are arranged in parallel laterally (side-by-side), definingparallel peaks and valleys (a facet is defined between each adjacentpeak and valley). In one embodiment, the lateral waviness is regularwith a constant or variable wavelength and/or wave amplitude (or extentof lateral deformation). The lateral waviness may generally follow asinusoidal profile, or other curved profile. In another embodiment, thelateral waviness may be of random wavelength and/or wave amplitude. Inone embodiment, the peaks are of constant or similar height and/or thevalleys are of constant or similar depth, across the plane of thesubstrate. The pitch between adjacent peaks/valleys may be constantacross a particular cross-sectional plane. In one embodiment, theoptical substrate includes a non-structured, smooth, planar, or glossylight input surface. In one embodiment, the light output surface and thelight input surface are generally parallel to each other in the overalloptical substrate structure (i.e., do not form an overall substratestructure that is generally tapered).

In another embodiment, the structured light output surface furtherincludes varying peak heights along each wavy prism structure in thestructured surface.

In a further embodiment, the structured light output surface furtherincludes, with or without varying peak heights, pre-defined structuralirregularities distributed in the structure surface. The pre-definedirregularities introduced may be in-kind to the anticipated structuraldefects, such as non-facet flat sections in the prism structure of thestructured surface.

The optical substrate may have a base portion, which may be a separatelayer from a layer bearing the structured surface, or is unitary ormonolithic to the prism structure of the structured surface. The baseportion provides the necessary thickness to provide structural integrityto the final luminance enhancement film.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of theinvention, as well as the preferred mode of use, reference should bemade to the following detailed description read in conjunction with theaccompanying drawings. In the following drawings, like referencenumerals designate like or similar parts throughout the drawings.

FIG. 1 schematically illustrates the structure of a prior art LCD.

FIGS. 2A-C illustrate prior art optical substrates having differentsurface structures.

FIGS. 3 to 5B illustrate prior art hybrid brightness enhancement opticalsubstrates.

FIG. 6 a is a schematic perspective view of an optical substrate havinga structured surface, in accordance with one embodiment of the presentinvention; FIGS. 6 b-6 f represent simulations of optical diffusioneffects along two orthogonal vertical planes of the optical substrate,at various degrees of lateral waviness.

FIG. 7 a is a schematic perspective view of a structured surface withvarying peak heights, in accordance with another embodiment of thepresent invention; FIGS. 7 b-7 f represent simulations of opticaldiffusion effects along two orthogonal vertical planes of the opticalsubstrate with varying heights.

FIG. 8 a is a schematic perspective view of a structured surface withdistributed predefined irregularities, in accordance with anotherembodiment of the present invention;

FIG. 8 b is a schematic perspective view of a structured surface withdistributed predefined irregularities, in accordance with yet anotherembodiment of the present invention;

FIGS. 8 c-8 g represent simulations of optical diffusion effects alongtwo orthogonal vertical planes of the optical substrate with distributedpre-defined irregularities.

FIGS. 9A to 9B illustrate SEM photos of prototype optical substrateshaving wavy prisms and distributed predefined irregularities inaccordance with a further embodiment of the present invention.

FIG. 10 schematically illustrates the structure of a LCD having anoptical substrate, which incorporate the optical substrate in accordancewith one embodiment of the present invention.

FIG. 11 is a schematic view of an electronic device comprising an LCDpanel that incorporates the inventive optical substrate of the presentinvention, in accordance with one embodiment of the present invention.

FIG. 12 is a schematic perspective view of an optical substrate havingan array of lateral row of uniform regular prisms.

FIG. 13 is a schematic perspective view of an optical substrate having acombination of structural features within the structured surface, inaccordance with one embodiment of the present invention.

FIG. 14 a illustrates a schematic three-dimensional diagram of the lightdirecting film in the present invention.

FIG. 14 b illustrates a top view of the light directing film in FIG. 14a.

FIG. 15 illustrates another embodiment of the light directing film inFIG. 14 a.

FIG. 16 a and FIG. 16 b illustrate another embodiment of the lightdirecting film in FIG. 14 a.

FIG. 17 a and FIG. 17 b illustrate another embodiment of the lightdirecting film in FIG. 14 a.

FIG. 18 illustrates the light directing film in FIG. 14 a combined withthe contexts and features in FIG. 17A to FIG. 17I, FIG. 18A to FIG. 18I,FIG. 19A to FIG. 19I and FIG. 20 of U.S. patent application Ser. No.14/729,029.

DETAIL DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The present description is of the best presently contemplated mode ofcarrying out the invention. This invention has been described herein inreference to various embodiments and drawings. This description is madefor the purpose of illustrating the general principles of the inventionand should not be taken in a limiting sense. It will be appreciated bythose skilled in the art that variations and improvements may beaccomplished in view of these teachings without deviating from the scopeand spirit of the invention. The scope of the invention is bestdetermined by referenced to the appended claims.

The present invention is directed to an optical substrate that possessesa structured surface that enhances luminance or brightness and reducesinterference fringes in the display images. In one aspect of the presentinvention, the optical substrate is in the form of a film, sheet, plate,and the like, which may be flexible or rigid, having a structured lightoutput surface that comprises rows of laterally arranged snaking, wavyor meandering longitudinal prism structures.

In the context of the present invention, the inventive optical substratemay be adopted in display devices having display panels that may be flator curved, and rigid or flexible, which comprise an array of displaypixels. Planar light source refers to a light source that providesillumination to cover an area of the array of display pixels.Accordingly, for display panels having a curved image plane of displaypixels (such panels may be rigid or flexible), the backlight would coverthe array of display pixels in the curved plane, to effectively provideillumination coverage to the curved image plane.

The present invention will be further described below in connection withthe illustrated embodiments.

FIG. 10 illustrates an example of a flat panel display. A backlight LCD100, in accordance with one embodiment of the present invention,comprises a liquid crystal (LC) display module 112, a planar lightsource in the form of a backlight module 114, and a number of opticalfilms interposed between the LC module 112 and the backlight module 114.The LC module 112 comprises liquid crystals sandwiched between twotransparent substrates, and control circuitry defining a two-dimensionalarray of pixels. The backlight module 114 provides planar lightdistribution, either of the backlit type in which the light sourceextends over a plane, or of the edge-lit type as shown in FIG. 10, inwhich a linear light source 116 is provided at an edge of a light guide118. A reflector 120 is provided to direct light from the linear lightsource 116 through the edge of the light guide 118 into the light guide118. The light guide is structured (e.g., with a tapered plate and lightreflective and/or scattering surfaces defined on the bottom surfacefacing away from the LC module 112) to distribute and direct lightthrough the top planar surface facing towards LC module 112. The opticalfilms may include optional upper and lower diffuser films 122 and 124that diffuse light from the planar surface of the light guide 118. It isnoted that while two diffuser films are shown in FIG. 10, given theoptical diffusion characteristic of the inventive optical substratesdisclosed in greater detail below, at least the upper diffuser film 122would not be necessary, in one embodiment of the present invention. Thelower diffuser 124 may also be omitted. This would reduce the overallthickness of the LCD 100. It is noted that a diffuser film or layer isdistinguishable from an optical substrate for brightness enhancement(i.e., brightness or luminance enhancement film discussed below), inthat the diffuser film does not have prismatic structures. Diffuser filmscatters and spreads light, instead of directing light to enhanceluminance in a direction out of the display. The optical substrate ofthe present invention has prismatic structures, which are configured toboth diffuse light and enhance luminance.

Specifically, the optical films shown in FIG. 6 a further include one ormore structured surface optical substrates in accordance with thepresent invention, which diffuse light as well as redistribute the lightpassing through such that the distribution of the light exiting thefilms is directed more along the normal to the surface of the films. Inthe illustrated embodiment, there are two structured optical substrates126 and 128 (which may be similar in structure) in accordance with thepresent invention, which are arranged with the longitudinal prismstructures generally orthogonal between the two substrates. Thestructured optical substrates 126 and 128 are structured to diffuselight as well as enhance luminance or brightness, redirecting light outof the display. The light entering the LC module 112 through such acombination of optical films is uniform spatially over the planar areaof the LC module 112 and has relatively strong normal light intensity.The structured optical substrate 126 and 128 obviate the need for aseparate diffuser sheet between the LC module 112 and the upperstructured optical substrate 126. Further, the structured opticalsubstrates 126 and 128 in accordance with the present invention wouldreduce interference fringe from being created between the substrates,and between the upper substrate and the adjacent LC module 112.Alternatively, only one of the optical substrates 126 and 128 need to bestructured in accordance with the present invention (e.g., only thelower optical substrate 128), to provide acceptable interference fringelevel and optical diffusion effect.

The optical substrates in accordance with the present invention may beused with LCDs to be deployed for displays, for example, fortelevisions, notebook computers, monitors, portable devices such as cellphones, digital cameras, PDAs and the like, to make the displaysbrighter.

While the backlight module 114 is shown with a light source 116 placedat an edge of the light guide plate 118, the backlight module may be ofanother light source configuration, such as an array of LEDs positionedat an edge of a light guide, or a planar array of LEDs in place of thelight guide plate, without departing from the scope and spirit of thepresent invention.

FIG. 6 a illustrates the structure of an optical substrate 50 inaccordance with one embodiment of the present invention, which can beused as the structured optical substrate 126 and/or 128 in FIG. 10. Theoptical substrate 50 has a light input surface 52 and a structured lightoutput surface 54 that includes prismatic structures that may be viewedas comprising parallel rows 56 of laterally meandering continuouslongitudinal prisms 58. The longitudinal prisms 58 meander laterally insubstantially smooth curves. In an alternate embodiment (not shown),curved segments (i.e., a section with a curve in a particular direction,or a C-shaped curve section) are coupled end-to-end, which form theoverall meandering longitudinal prism structures. In the illustratedembodiment, the light input surface 52 is non-structured, smooth,planar, and/or glossy. It is understood that the light input surface 52may be textured (e.g., a frosted or matte finish, or particles dispersedat the surface; see U.S. patent application Ser. No. 12/832,021, filedJul. 7, 2010, which is fully incorporated by reference herein). In theillustrated embodiment, the light output surface and the structuredlight input surface are generally parallel to each other in the overalloptical substrate structure (i.e., do not form an overall substratestructure that is generally tapered like a light guide plate in abacklight module, or that is concave or convex).

In the embodiment of FIG. 6 a, the laterally meandering rows 56 oflongitudinal prisms 58 are arranged in parallel laterally(side-by-side), defining parallel peaks 60 and valleys 62. A wavy facetis defined between each adjacent peak 60 and valley 62. In theillustrated embodiment of FIG. 6 a, the lateral waviness is regular witha constant wavelength and/or wave amplitude (i.e., extent of lateraldeformation), generally following a sinusoidal profile. The lateralwaviness may follow other curved profiles, which may be irregular and/orrandom in wavelength and/or wave amplitude (or lateral deformation) (seethe embodiment shown in FIG. 9). The peak vertex angle may be rightangled, and the peaks are of constant or similar height and/or thevalleys are of constant or similar depth, across the plane of thesubstrate. The prisms 58 each has a constant sectional profile in thex-z plane. In other embodiments, the lateral waviness may be irregular,further with a variable wavelength and/or wave amplitude.

The distance or pitch between adjacent peaks/valleys is constant acrossa particular x-z sectional plane. In this embodiment, the light outputsurface and the structured light input surface are generally parallel toeach other in the overall optical substrate structure (i.e., do not forman overall substrate structure that is generally tapered like a lightguide plate in a backlight module, or that is concave or convex).

For ease of reference, the following orthogonal x, y, z coordinatesystem will be adopted in explaining the various directions. As shown inFIG. 6 a, the x-axis is in the direction across the peaks and valleys,also referred to as the lateral or transverse direction. The y-axis isorthogonal to the x-axis, in a generally longitudinal direction of theprisms 58. The longitudinal direction of prisms would be in reference tothe general direction in which the peaks 60 proceed from one end toanother end of the prisms 58, wherein the prisms meander about they-axis. The light input surface 52 lies in an x-y plane. For arectangular piece of the optical substrate, the x and y-axes would bealong the orthogonal edges of the substrate. The z-axis is orthogonal tothe x and y-axes. The edge showing the ends of the laterally arrangedmeandering rows 56 of prisms 58 blocks lies in the x-z plane, such asshown in FIG. 6 a, which also represents a sectional view in the x-zplane. References to cross sections of prisms 58 would be sections takenin x-z planes, at various locations along the y-axis. Further,references to a horizontal direction would be in an x-y plane, andreferences to a vertical direction would be in the z-direction.

In the illustrated embodiment, the substrate 50 comprises two separatelayers, wherein the top structured surface layer 68 has the structuredlight output surface 54, and the bottom base layer 66 has the planarlight input surface 52. The two layers are adhered together to form thesubstrate 50. It can be appreciated that the substrate may be formedfrom a single integrated physical layer of material, instead of twoseparate physical layers, without departing from the scope and spirit ofthe present invention. The optical substrate 50 may be a unitary ormonolithic structure including a base portion integral to the prismstructures that define the structured surface.

In the illustrated embodiment, the structured surface layer 68 and thebase layer 66 are made of different materials. The structured surfacelayer 68 may be formed using an optically transparent material,preferably a polymerizable resin, such an ultraviolet or visibleradiation-curable resin. Generally, the structured surface 54 is formedby applying a coatable composition comprising a polymerizable andcrosslinkable resin onto a master mold or master drum and undergoing ahardening process. The base layer 66 may be made of PET (polyethyleneterephthalate) material, but may be made from the same transparentmaterial as the structured layer 68. The base layer 66 provides thenecessary thickness to provide structural integrity to the final opticalluminance enhancement film.

The dimensions of the structured surface are generally as follows, forexample:

Thickness of base layer=tens of micrometers to several millimeters

Peak height (as measured from the top of base layer)=tens to hundreds ofmicrometers

Distance of valley bottom from top of base layer=about 0.5 to hundredsof micrometers

Vertex angle of peaks=about 70 to 110 degrees

Pitch between adjacent peaks=tens to hundreds of micrometers

Wavelength W of lateral wavy prisms=tens of micrometers to severalmillimeters

Lateral deformation D (i.e., twice amplitude of lateral wavyprisms)=several to hundreds of micrometers

FIGS. 6 b to 6 f described hereafter respectively represents the resultsof the simulations with lower optical substrates 50 having wavy prismsof a constant wavelength W=100 μm and deformations D=0, 10, 20, 30 and40 μm (i.e. D/W=0, 0.1, 0.2, 0.3 and 0.4), wherein the wavelength W andthe lateral deformation D is defined in FIG. 6 a. However, thewavelength W and the lateral deformation D in FIG. 6 a can be alsodefined in the following way. FIG. 14 a illustrates a schematicthree-dimensional diagram of the light directing film 200 in the presentinvention (For convenience of description, only two prism elements 211,212 are shown). FIG. 14 b illustrates a top view of the light directingfilm 200 in FIG. 14 a. The light directing film 200 comprises a firststructured major surface 201, a second major surface 202 opposite to thefirst structured major surface 201, wherein the first structured majorsurface 201 comprises a first prism element 211 and a second prismelement 212 meandering in a wary manner. Preferably, the first prismelement 211 and the second prism element 212 meander in a smooth warymanner. Each of the first prism element 211 and the second prism element212 meanders in the form of a wave, wherein the wave has a wavelengthdirection (e.g., Y axis), an amplitude direction (e.g., X axis) and acentral axis 215 extending in the wavelength direction such that each ofthe first prism element 211 and the second prism element 212 has anaverage amplitude A relative to the central axis 215 in the amplitudedirection. Specifically speaking, each of the first prism element 211and the second prism element 212 has an average amplitude A relative tothe central axis 215 in the amplitude direction and an averagewavelength λ in the wavelength direction. In this case, the ridge (orpeak line) 211A, 212A of each of the first prism element 211 and thesecond prism element 212 meanders with respective to the central axis215. The valley line 217 between the adjacent prism elements 211, 212can be parallel to the ridges 211A, 212A. The average amplitude A can bein the range of one of 0˜20 μm, 5˜20 μm, 10˜20 μm, 5˜15 μm, 5˜10 μm,10˜15 μm and 15˜20 μm. Comparing the wavelength W and the lateraldeformation D defined in FIG. 6 a to the average amplitude A and theaverage wavelength λ defined in FIG. 14 b, the wavelength W defined inFIG. 6 a is substantially equal to the average wavelength λ defined inFIG. 14 b and the lateral deformation D defined in FIG. 6 a is twice theaverage amplitude A defined in FIG. 14 b, so that the ratio of theaverage amplitude A to the average wavelength λ can be 0, 0.05, 0.1,0.15 and 0.2, in other words, the ratio of the average amplitude A tothe average wavelength λ can be in the range of one of 0˜0.2, 0.05˜0.2,0.1˜0.2, 0.05˜0.15, 0.05˜0.1, 0.1˜0.15 and 0.15˜0.2.

In another embodiment, the structured light output surface of opticalsubstrate further includes varying peak heights along each wavy prismstructure in the structured surface, in addition to the laterally wavyprism structure (see FIG. 13; see also FIG. 7 a). The peak height mayvary in an orderly, semi-orderly, random, and quasi-random manner. FIG.7 a illustrates peak variations in a regular, orderly manner, followinga generally sinusoidal waveform.

In a further embodiment, the structured light output surface furtherincludes, with or without varying peak heights, predefined structuralirregularities distributed in the structure surface. The pre-definedirregularities introduced may be in-kind to anticipated structuraldefects arising from manufacturing, such as non-facet flat sections inthe prism structure (e.g., at peaks or valleys) of the structuredsurface (see FIG. 9 and FIG. 13; see also FIG. 8 a). The structuralirregularities are distributed across the structured light outputsurface in at least one of orderly, semi-orderly, random, andquasi-random manner. The predefined irregularities introduced into thestructured light output surface could mask certain user perceivabledefects caused by structural defects that have been unintentionallyincluded in the structured light output surface from the manufacturingprocess. Further reference to the defect masking effect of predefinedstructural irregularities may be made to U.S. application Ser. No.11/825,139 filed on Jul. 2, 2007, which has been fully incorporated byreference herein.

Computer Simulation of Optical Diffusion Effects

Computer simulation model have been undertaken for trend analysis ofoptical diffusion effects, comprising only two crossed brightnessenhancement optical substrates in accordance with the present invention.Generally, for purpose of trend analysis, as more fully detailed below,only one of the substrates is structured with the wavy prism, varyingprism peak heights or flat irregularities. With only one of the twosubstrates as structure, the effect of the structured surfaces would bemore easily determined. The upper substrate is structured with straighttriangular prisms on one side, and a glossy or smooth surface on theother side. The lower substrate is structured using only one of wavyprism, varying prism peak heights and flat irregularities. Thestructured surface of the lower substrate is adjacent to the glossy sideof the upper substrate, and the other side of the lower substrate isglossy or smooth. The light source inputs from the smooth light inputsurface of the lower substrate. The simulation model is thus simplifiedand used to get the optical diffusion distribution trend of output lightfrom the upper substrate. No reflector, light guide or other componentshas been specifically considered.

Computer simulations have been conducted to investigate the opticaldiffusion effects along the x-z plane and the y-z plane of opticalsubstrates 50 having different degree of waviness (i.e., differentdegrees of lateral deformation D) at a constant wavelength W of 100 μm.The simulations were conducted with a combination of an upper opticalsubstrate 70 (see FIG. 12) having an array of lateral rows of straight,uniform, regular prisms 71, and a lower optical substrate having thestructure of the optical substrate 50, wherein the upper substrate 70and the lower substrate 50 are rotated 90 degrees about the z-axis, sothat the x-axis of the upper substrate 70 is aligned with the y-axis ofthe lower substrate 50. The underside of the upper substrate 70 thatfaces the structured surface of the lower substrate 50 is smooth. Theupper substrate 70 has peaks that are 50 μm in pitch and 90 degrees invertex angle. The lower substrate has similar peak pitch and vertexangle. Lambertian light is directed to the light input surface at thebottom of the lower substrate. With only one of the two opticalsubstrates as structured with lateral wavy prism structures (or varyingpeak heights and flat irregularities in the other simulations), theoptical diffusion effect of the film structures would be more easilydetermined.

FIGS. 6 b to 6 f respectively represents the results of the simulationswith lower optical substrates 50 having wavy prisms of a constantwavelength W=100 μm and deformations D=0, 10, 20, 30 and 40 μm. The leftside of FIGS. 6 b to 6 f represents the optical diffusion effects alongthe x-z plane of the lower substrate 50 having the inventive prismstructure shown in FIG. 6 a; the right side of FIGS. 6 b to 6 frepresents the optical diffusion effects along the y-z plane of thelower substrate 50 having the inventive wavy prism structures shown inFIG. 6 a. Based on simulation results, one can see a clear trend in theoptical diffusion effects, wherein the diffused output lightdistribution from the upper substrate 70 significantly improves inuniformity with higher deformation D. The simulation results show thatthe diffused light from output surface is increased rapidly along both xand y directions with increasing transverse deformations D. The outputlight is diffused more in lateral or transverse direction (x-z plane)than that in the longitudinal direction (y-z plane). With deformationD=0 (FIG. 6 b), the output light is more concentrated and significantlyless diffused.

For purpose of simulation trend analysis of the effect of vary peakheights, FIG. 7 a schematically illustrates an optical substrate 72having only varying peak heights along each straight longitudinal prismstructure. The peak height of the prisms varies with variance V.

Computer simulations have been conducted to investigate the opticaldiffusion effects along the x-z plane and the y-z plane of opticalsubstrates 72 having different degrees of peak height variance V. As inthe previous embodiment, the simulations were conducted with acombination of an upper optical substrate 70 (see FIG. 12) having anarray of lateral rows of straight, uniform, regular prisms 71, and alower optical substrate having the structure of the optical substrate72, wherein the upper substrate 70 and the lower substrate 72 arerotated 90 degrees about the z-axis, so that the x-axis of the uppersubstrate 70 is aligned with the y-axis of the lower substrate 72. Thesimulation conditions are otherwise similar to the previous simulationsin FIGS. 6 b to 6 f. The underside of the upper substrate 70 that facesthe structured surface of the lower substrate 72 is smooth. The uppersubstrate 70 has peaks that are 50 μm in pitch and 90 degrees in vertexangle. The lower substrate 72 has similar peak pitch and vertex angle.Lambertian light is directed to the light input surface at the bottom ofthe lower substrate.

FIGS. 7 b to 7 f respectively represents the results of the simulationswith lower optical substrates 72 having peak height variances V=0, 10,20, 30 and 40 μm. The left side of FIGS. 7 b to 7 f represents theoptical diffusion effects along the x-z plane of the lower substrate 72having the inventive prism structure shown in FIG. 7 a; the right sideof FIGS. 7 b to 7 f represents the optical diffusion effects along they-z plane of the lower substrate 72 having the inventive prism structureshown in FIG. 7 a. Based on the simulation results, one can see that theoptical diffusion effects do not change significantly with increasingpeak height variance V, wherein the diffused output light distributionfrom the upper substrate 70 increased only slightly in uniformity withhigher peak height variance V. The simulation results show that thediffused light from output surface does not change significantly alongboth x and y directions with increasing peak height variance V. Theoutput light remains concentrated and less diffused with changes in peakheight variance V.

However, for purpose of simulation trend analysis of the effect of flatirregularities, FIG. 8 a schematically illustrates an optical substrate76 having only predefined irregularities 78 distributed in the prismstructure of optical substrate 76. In order to simplify the simulationmodel, the structure is relocated as straight regular longitudinalprisms 84 with flat gaps 82 between adjacent prisms 84. The ratio R of bto a (shown in FIG. 8 b) is used to control the area percentage of theflat irregularities to whole area. FIGS. 8 c to 8 g exhibit the trend ofoptical diffusion effects of structures with ratio R=0, 2.5, 5, 10 and20% respectively.

Computer simulations have been conducted to investigate the opticaldiffusion effects along the x-z plane and the y-z plane of opticalsubstrates 80 having different degree of ratio R. As in the previousembodiment, the simulations were conducted with a combination of anupper optical substrate 70 (see FIG. 12) having an array of lateral rowsof straight, uniform, regular prisms 71, and a lower optical substratehaving the structure of the optical substrate 80, wherein the uppersubstrate 70 and the lower substrate 80 are rotated 90 degrees about thez-axis, so that the x-axis of the upper substrate 70 is aligned with they-axis of the lower substrate 80. The simulation conditions areotherwise similar to the previous simulations in FIGS. 6 b to 6 f. Theunderside of the upper substrate 70 that faces the structured surface ofthe lower substrate 50 is smooth. The upper substrate 70 has peaks thatare 50 μm in pitch and 90 degrees in vertex angle. The lower substrate80 has similar peak pitch and vertex angle. Lambertian light is directedto the light input surface at the bottom of the lower substrate.

FIGS. 8 c to 8 g respectively represents the results of the simulationswith lower optical substrates 80 having ratio R=0, 2.5, 5, 10 and 20%.The left side of FIGS. 8 c to 8 g represents the optical diffusioneffects along the x-z plane of the lower substrate 80 having theinventive prism structure shown in FIG. 8 b; the right side of FIGS. 8 cto 8 g represents the optical diffusion effects along the y-z plane ofthe lower substrate 80 having the inventive prism structure shown inFIG. 8 b. Based on the simulation results, one can see that the opticaldiffusion effects do not change appreciably with increasing ratio R,wherein the diffused output light distribution from the upper substrate70 hardly change in uniformity with higher ratio R. The simulationresults show that the diffused light from output surface does not changeappreciably along both x and y directions with increasing ratio R. Theoutput light remains at same level of being concentrated and lessdiffused with changes in ratio R.

Based on the foregoing trend analysis for the different approaches ofwavy prisms, peak height variation, and flat irregularities, whereineach approach is considered alone separately, the following opticaldiffusion effects were observed. The overall diffused light from theoutput surface of the upper optical substrate is rapidly increased whileincreasing the transverse deformation D of the wavy prisms. The overalldiffused light from the output surface of the upper optical substrate isincreased slightly while increasing the peak height variance V.Accordingly, the transverse deformation D of the wavy prisms has a moresignificant effect on diffusing light. Ratio R of flat irregularitieshas least influence compared with transverse deformation and peak heightvariance. Given the foregoing diffusion analysis, one can anticipate theeffects of combining the different approaches to reduceinterference-fringes without compromising diffusion.

The foregoing simulations did not consider the effect of randomly orregularly arranging a combination of different degrees of transversedeformation D, peak height variance V and ratio R of flat regularities.All the simulated structures are parallel between prisms. It isreasonable to predict that the diffusion effects will be enhanced, ifthe transverse deformation D, peak height variance V and ratio of flatirregularities R are appropriately applied in location and magnitude.

Experimental Results

Prototype optical substrates in accordance with the present inventionhave been made that included combinations of transverse prismdeformation, peak variance and irregularities, which are computed andwell distributed in position and magnitude. FIG. 9 a is a SEM photo thatshows the wavy prisms and flat irregularities with different sizes. FIG.9 b is a magnified photo of FIG. 9 a. FIG. 13 is a schematic perspectiveview of an optical substrate 77, having laterally wavy prisms (as inFIG. 6 a), prism peak height variation (section 79, as in FIG. 7 a) andflat irregularities 78 (as in FIG. 8 a).

While observing the interference fringes, an upper optical enhancementsubstrate with straight prism structures (no wavy prism deformation,variance in peak height or flat irregularities) on one side and with aglossy surface on the other side (opposite to the structured surface),is applied. The upper straight prism substrate is stacked above theprototypes of the present invention. Each lower substrate prototype inaccordance with the present invention is crossed stacked with the upperstraight prism substrate, as was in the computer simulation arrangement.These stacked substrates are illuminated by a backlight as shown in FIG.10. The interference fringes (bright and dark repeated patterns) areobserved from top, light output surface, of the upper straight prismsubstrate.

Table 1 shows the performance of 9 embodiments of lower substrateprototypes. The interference fringe level is in reference to level 5 ofinterference fringes (on a scale of 0 to 5) that are present in thereference case of two straight prism enhancement films stacked crossly.Gain is the ratio of luminance of a single embodiment to that of thelight guide.

Embodiments 4 and 8 reduce the level from 5 to 1 and 1.5. The otherembodiments all eliminate the interference fringes. Embodiment 4 showsthat 12 μm deformation mixing with 10.4% flat irregularities can givesignificant improvement and better performance on interference fringesthan that of embodiment 8 which has 25 μm deformation without anyirregularities. This indicates that irregularities are useful indiffusing light for eliminating the fringes. However, embodiments 1 to 4shows that the bigger area of flat irregularities, the lower the gain.In order to maintain gain without acceptable loss, the flatirregularities should be arranged specifically and the range of flatirregularities should be controlled for different requirements ofvarious applications. In general, the effects of the flat irregularitieson interference fringes depend on the total area of the flatirregularities, the number, shapes, sizes and locations of the flatirregularities.

TABLE 1 Wavy Prism variance in Transverse Deformation peak flat (%)Interference embodiment Ave. (μm) Max (μm) height (μm) irregularitiesfringe level gain 1 25 55 <10 2.1 0 1.55 2 24 48 <10 4.3 0 1.51 3 26 39<10 8.3 0 1.46 4 12 21 <10 10.4 0 1.46 5 29 54 <10 1.1 0 1.54 6 28 53<10 2.5 0 1.54 7 30 55 <10 0.1 0 1.54 8 25 38 <10 0 1.5 1.55 9 28 45 <50 0 1.54

It is not necessary to have the combination of wave transversedeformation, peak height variance and ratio of irregularities to provideacceptable reduction in interference fringes. For example, the opticalsubstrate may have only transverse deformation without variance in peakheight and irregularities. As demonstrated by embodiment 9, a transversedeformation of 28 μm and a small peak height variance, can stilleliminate interference fringes without adopting any irregularities.Also, it is anticipated that the effect of eliminating interferencefringes still exists, if the peak height variance is reduced to zero,with the presence of at least some flat irregularities.

The structured surface of article of the present invention may begenerated in accordance with a number of process techniques, includingmicromachining using hard tools to form molds or the like for theirregular prismatic profile described above. The hard tools may be verysmall diamond tools mounted on CNC (Computer Numeric Control) machines(e.g. turning, milling and ruling/shaping machines). Preferably thesemachines may add some vibration or perturbation generating devices toassist the tools moving with small shifts and making prisms withdifferent level of irregularity. Known STS (Slow Tool Servo), FTS (FastTool Servo) and some ultrasonic vibration apparatus are exemplarydevices. U.S. Pat. No. 6,581,286, for instance, discloses one of theapplications of the FTS for making grooves on an optical film by usingthread cutting method. The tool is mounted onto the machine, to createconstant peak vertex angle in relation to x-z planes along the ydirection within a prism. By using the devices to form surfaces in themold in relation to increasing degrees of freedom, the structuredsurfaces of the optical substrate disclosed above can be obtained.

The master may be used to mold the optical article directly or used inelectroforming a duplicate of the master, which duplicate is used tomold the optical article. The mold may be in the form of a belt, a drum,a plate, or a cavity. The mold may be used to form the prismaticstructure on a substrate through hot embossing of the article, and/orthrough the addition of an ultraviolet curing or thermal settingmaterials in which the structures are formed. The mold may be used toform the optical article through injection molding. The substrate orcoating material may be any organic, inorganic or hybrid opticallytransparent material and may include suspended diffusion, bi-refringentor index of refraction modifying particles.

Further discussions of processes for forming a substrate havingstructured surfaces may be referenced to U.S. Pat. No. 7,618,164, whichhad been incorporated by reference herein.

In accordance with the present invention, the optical substrate (e.g.,50, 72, 80 and 77) comprises a prismatic, structured light outputsurface having a combination of laterally meandering longitudinalprisms, predefined, intentionally introduced irregularities, and/orprism peak variations, which together enhances brightness, reducesinterference fringes, and masks otherwise user perceivable defects, whenapplied in an LCD for example. An inventive LCD incorporating theinventive optical substrate in accordance with the present invention maybe deployed in an electronic device. As shown in FIG. 11, an electronic110 (which may be one of a PDA, mobile phone, television, displaymonitor, portable computer, refrigerator, etc.) comprises the inventiveLCD 100 in accordance with one embodiment of the present invention. TheLCD 100 comprises the inventive optical substrate described above. Theelectronic device 110 may further include within a suitable housing, auser input interface such as keys and buttons (schematically representedby the block 116), image data control electronics, such as a controller(schematically represented by block 112) for managing image data flow tothe LCD 100, electronics specific to the electronic device 110, whichmay include a processor, A/D converters, memory devices, data storagedevices, etc. (schematically collectively represented by block 118), anda power source such as a power supply, battery or jack for externalpower source (schematically represented by block 114), which componentsare well known in the art.

FIG. 15 illustrates another embodiment of the light directing film 200in FIG. 14 a. The distance between the ridge 211A of the first prismelement 211 and the ridge 212A of the second prism element 212 variesalong the length direction of the first prism element 211 and the secondprism element 212, which can be shown in FIG. 2, FIG. 3, FIG. 5, FIG. 7,FIG. 8, FIG. 10, FIG. 11, FIG. 13 and FIG. 14 of U.S. Pat. No. 7,618,164in CROSS-REFERENCE TO RELATED APPLICATIONS.

FIG. 16 a illustrates another embodiment of the light directing film 200in FIG. 14 a. A reference plane 203 (e.g., a hypothetical plane whichcan be arbitrarily selected) is between the first structured majorsurface 201 and the second major surface 202. The reference plane 203can be also selected from one of the second major surface 202 and theinterface between the prism layer and the substrate supporting the prismlayer (not shown). The reference plane 203 is substantiallyperpendicular to the thickness direction (e.g., Z axis) of the lightdirecting film 200. Each of the first prism element 211 and the secondprism element 212 comprises: a first portion 231, wherein a first ridgeof the first portion 231 has a constant height 231B relative to thereference plane 203; and a second portion 241 adjacent to the firstportion 231, wherein a second ridge of the second portion 241 has anon-constant height 241B relative to the reference plane 203, whereinthe maximum of the non-constant height 241B of the second portion 241 islarger than the constant height 231B of the first portion 231.

The first portion 231 has a constant bottom width 231X and the secondportion 241 has a non-constant bottom width 241X, wherein the maximum ofthe non-constant bottom width 241X of the second portion 241 is largerthan the constant bottom width 231X of the first portion 231 (see FIG.16 b). The first portion 231 has a constant cross-sectional shape andthe second portion 241 has a non-constant cross-sectional shape, eachedge 241X, 241Y, 241Z of the maximum of the non-constant cross-sectionalshape of the second portion 241 is respectively enlarged from acorresponding edge 231X, 231Y, 231Z of the cross-sectional shape of thefirst portion 231 by a ratio larger than 1 (see FIG. 16 b).

FIG. 17 a illustrates another embodiment of the light directing film 200in FIG. 14 a. A reference plane 203 (e.g., a hypothetical plane whichcan be arbitrarily selected) is between the first structured majorsurface 201 and the second major surface 202. The reference plane 203can be also selected from one of the second major surface 202 and theinterface between the prism layer and the substrate supporting the prismlayer (not shown). The reference plane 203 is substantiallyperpendicular to the thickness direction (e.g., Z axis) of the lightdirecting film 200. Each of the first prism element 211 and the secondprism element 212 comprises: a first portion 251, wherein a first ridgeof the first portion 251 has a first constant 251B height relative tothe reference plane 203; and a second portion 261, wherein a secondridge of the second portion 261 has a second constant height 261Brelative to the reference plane 203, wherein the second constant height261B of the second portion 261 is larger than the first constant height251B of the first portion 251.

The first portion 251 has a first constant bottom width 251X and thesecond portion 261 has second constant bottom width 261X, wherein thesecond constant bottom width 261X of the second portion 261 is largerthan the first constant bottom width 251X of the first portion 251 (seeFIG. 17 b). The first portion 251 has a first constant cross-sectionalshape and the second portion 261 has a second constant cross-sectionalshape, each edge 261X, 261Y, 261Z of the second constant cross-sectionalshape of the second portion 261 is respectively enlarged from acorresponding edge 251X, 251Y, 251Z of the first constantcross-sectional shape of the first portion 251 by a ratio larger than 1(see FIG. 17 b).

FIG. 17A to FIG. 17I, FIG. 18A to FIG. 18I, FIG. 19A to FIG. 19I andFIG. 20 of U.S. patent application Ser. No. 14/729,029 illustrativelydescribes the prism elements extending substantially in a firstdirection (e.g., Y axis). However, the contexts and features aboutdescribing “the ridges (or peaks, apexes) of the taller prism elementsof the first optical sheet limit the proximity of the second opticalsheet to the top structured major surface of the first optical sheet inorder to reduce the likelihood of wet-out”, “the height differencebetween the taller prism elements and shorter prism elementssignificantly inhibits the occurrence of undesired optical coupling inthe zone of shorter prism elements” and “using the top structured majorsurface of the first optical sheet to control the proximity dramaticallyreduces the surface area of the top structured major surface which issusceptible to undesired optical coupling” in FIG. 17A to FIG. 17I, FIG.18A to FIG. 18I, FIG. 19A to FIG. 19I and FIG. 20 of U.S. patentapplication Ser. No. 14/729,029 can be applied to the prism elementsmeandering in a wary manner in the present invention, such as the tallerprism element 311 and the shorter prism element 312 in FIG. 18.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed structures andprocesses of the present invention without departing from the scope orspirit of the invention. In view of the foregoing descriptions, it isintended that the present invention covers modifications and variationsof this invention if they fall within the scope of the following claimsand their equivalents.

What is claimed is:
 1. A light directing film comprising a first structured major surface, a second major surface opposite to the first structured major surface, wherein the first structured major surface comprises a first prism element and a second prism element meandering in a wary manner.
 2. The light directing film according to claim 1, wherein each of the first prism element and the second prism element meanders in the form of a wave, wherein the wave has a wavelength direction, an amplitude direction and a central axis extending in the wavelength direction such that each of the first prism element and the second prism element has an average amplitude A relative to the central axis in the amplitude direction, wherein 0 μm<A<20 μm.
 3. The light directing film according to claim 2, wherein 5 μm<A<20 μm.
 4. The light directing film according to claim 2, wherein 10 μm<A<20 μm.
 5. The light directing film according to claim 2, wherein 5 μm<A<15 μm.
 6. The light directing film according to claim 1, wherein each of the first prism element and the second prism element meanders in the form of a wave, wherein the wave has a wavelength direction, an amplitude direction and a central axis extending in the wavelength direction such that each of the first prism element and the second prism element has an average amplitude A relative to the central axis in the amplitude direction and an average wavelength λ in the wavelength direction, wherein 0<A/λ<0.2.
 7. The light directing film according to claim 6, wherein 0.05<A/λ<0.2.
 8. The light directing film according to claim 6, wherein b 0.1<A/λ<0.2.
 9. The light directing film according to claim 6, wherein 0.05<A/λ<0.15.
 10. The light directing film according to claim 1, wherein the distance between a first ridge of the first prism element and a second ridge of the second prism element varies along the length direction of the first prism element and the second prism element.
 11. The light directing film according to claim 1, wherein the first prism element and the second prism element meander in a smooth wary manner.
 12. The light directing film according to claim 1, further comprising a reference plane between the first structured major surface and the second major surface, wherein the reference plane is substantially perpendicular to the thickness direction of the light directing film, wherein each of the first prism element and the second prism element comprises: a first portion, wherein a first ridge of the first portion has a constant height relative to the reference plane; and a second portion adjacent to the first portion, wherein a second ridge of the second portion has a non-constant height relative to the reference plane, wherein the maximum of the non-constant height of the second portion is larger than the constant height of the first portion.
 13. The light directing film according to claim 12, wherein the first portion has a constant bottom width and the second portion has a non-constant bottom width, wherein the maximum of the non-constant bottom width of the second portion is larger than the constant bottom width of the first portion.
 14. The light directing film according to claim 12, wherein the first portion has a constant cross-sectional shape and the second portion has a non-constant cross-sectional shape, each edge of the maximum of the non-constant cross-sectional shape of the second portion is respectively enlarged from a corresponding edge of the cross-sectional shape of the first portion by a ratio larger than
 1. 15. The light directing film according to claim 1, further comprising a reference plane between the first structured major surface and the second major surface, wherein the reference plane is substantially perpendicular to the thickness direction of the light directing film, wherein each of the first prism element and the second prism element comprises: a first portion, wherein a first ridge of the first portion has a first constant height relative to the reference plane; and a second portion, wherein a second ridge of the second portion has a second constant height relative to the reference plane, wherein the second constant height of the second portion is larger than the first constant height of the first portion.
 16. The light directing film according to claim 15, wherein the first portion has a first constant bottom width and the second portion has second constant bottom width, wherein the second constant bottom width of the second portion is larger than the first constant bottom width of the first portion.
 17. The light directing film according to claim 15, wherein the first portion has a first constant cross-sectional shape and the second portion has a second constant cross-sectional shape, each edge of the second constant cross-sectional shape of the second portion is respectively enlarged from a corresponding edge of the first constant cross-sectional shape of the first portion by a ratio larger than
 1. 18. The light directing film according to claim 11, further comprising a reference plane between the first structured major surface and the second major surface, wherein the reference plane is substantially perpendicular to the thickness direction of the light directing film, wherein a first ridge of the first prism element has a first height relative to the reference plane and a second ridge of the second prism element has a second height relative to the reference plane, wherein the first height of the first ridge of the first prism element varies along the length direction thereof, wherein the maximum of the first height is larger than the maximum of the second height.
 19. The light directing film according to claim 1, further comprising a reference plane between the first structured major surface and the second major surface, wherein the reference plane is substantially perpendicular to the thickness direction of the light directing film, wherein a first ridge of the first prism element has a first height relative to the reference plane and a second ridge of the second prism element has a second height relative to the reference plane, wherein the first height of the first ridge of the first prism element varies along the length direction of the first prism element, wherein the first ridge of the first prism element comprises: a first portion, wherein the first height of the first portion has a constant value; and a second portion adjacent to the first portion, wherein the first height of the second portion has a non-constant value, wherein the maximum of the non-constant value of the first height of the second portion is larger than the constant value of the first height of the first portion; wherein the maximum of the non-constant value of the first height of the second portion is larger than the maximum of the second height.
 20. The light directing film according to claim 1, further comprising a reference plane between the first structured major surface and the second major surface, wherein the reference plane is substantially perpendicular to the thickness direction of the light directing film, wherein a first ridge of the first prism element has a first height relative to the reference plane and a second ridge of the second prism element has a second height relative to the reference plane, wherein the first height of the first ridge of the first prism element varies along the length direction of the first prism element, wherein the first ridge of the first prism element comprises: a first portion, wherein the first height of the first portion has a first constant value; and a second portion, wherein the first height of the second portion has a second constant value, wherein the second constant value is larger than the first constant value; wherein the second constant value is larger than the maximum of the second height. 